The present disclosure relates to stops commonly used on orthodontic archwires in combination with tooth-mounted orthodontic brackets for treatment of tooth alignment issues. More particularly, the present disclosure relates to stops that define a non-linear path for an archwire to minimize sliding on the archwire, archwire assemblies incorporating such stops pre threaded on the archwire and methods of manufacturing archwire assemblies incorporating such stops.
Orthodontic treatment normally involves the application of mechanical forces to urge improperly positioned teeth into correct alignment. One common form of orthodontic treatment includes the use of orthodontic brackets that are fixed to teeth commonly by adhesively bonding the brackets directly to the teeth. A resilient curved archwire is then seated in the archwire slots of the brackets to impart mechanical forces to the teeth via the bracket. In traditional orthodontic treatment, the archwires may be secured to the brackets by ligature wires or elastic bands, which can limit relative movement between the archwire and the brackets. It has been found that free movement of the archwire relative to orthodontic brackets facilitates tooth movement, which is a goal of orthodontic treatment. Brackets of the self-ligating type were developed to eliminate the need for wires or elastic ligatures in securing archwires to orthodontic brackets and permit greater freedom of relative movement between the archwire and the brackets.
Brackets of the self-ligating type include a movable cover that selectively closes the archwire slot of the brackets to secure the archwire to the bracket, eliminating the need for ligature wires or elastic bands. The movable cover is opened for inserting the archwire and then closed for retaining the archwire within the archwire slot. The archwire is elastically deformed to engage the brackets, and seeks to return to its designed curve, thereby imparting mechanical force that urges the teeth to move to the correct position over time. Once secured in the archwire slot by the cover, the archwire is free to move laterally in the archwire slot, which facilitates tooth movement during treatment.
The enhanced freedom of movement of the archwire relative to self-ligating brackets can result in undesirable migration of the archwire from its intended installed position. Unbalanced forces produced by the tongue, mouth muscles and chewing will move the archwire laterally through the archwire slots of the brackets. This movement may cause a free end of the archwire to protrude from one of the brackets attached to the molars and contact gum or cheek tissue. As a result of the movement, the opposite free end of the archwire may also become disengaged from its bracket. The protruding ends of the archwire can irritate the gum or cheek tissue. Further, orthodontic treatment is disrupted by release of the archwires from the brackets.
Several conventional techniques are used to limit movement of the archwire in the bracket slots to prevent disengagement of the archwire from the brackets as well as to direct forces to one or more teeth. One technique is to insert the archwire through a crimpable sleeve, such as a small diameter tube, then position the archwire within the bracket slots with the sleeve located between two adjacent brackets. The sleeve is then secured (crimped) to the archwire at a fixed position to form a stop. The sleeve is configured such that the sleeve cannot pass through or move beyond an archwire slot as the archwire moves in the lateral direction. In this manner, the maximum movement of archwire is limited to somewhat less than the distance between the adjacent brackets. This arrangement effectively prevents the free ends of the archwire from becoming disengaged from the molars at the back of the mouth while permitting free movement of the archwire relative to the bracket.
There are inherent complications with the use of stops in the clinical setting. The principle problem is their very small size. Typical stops are about 10 to 30% of the size of an orthodontic bracket. Tubes used for stops are often 0.03″ to 0.04″ in diameter and only 0.08″ long. Tubes are mounted on the archwire in clinical settings such as a doctor's office, usually by the dentist or a dental assistant. It can be a challenge to see and handle these very small components, which then tend to slide freely along the archwire under their own weight. These small tubes may either slide to the wrong location for treatment or slide off the wire completely after being threaded onto the archwire but prior to installation being completed. A similar complication occurs when the clinician uses multiple stops on a single wire and must control the stop location as the archwire is being placed in the patient's mouth. FIG. 2 illustrates a prior art stop mounted on an archwire, where the stop is free to slide along the archwire as described above.
It is known to provide assemblies with tubes (stops) that are pre-mounted on archwires. One common method is to deform (partially crimp) the stops against the archwire (also termed ‘flattening’) so as to limit the sliding motion and thereby prevent the stop from falling off of the archwire. Since archwires are typically curved in a flat plane, is intuitive to flatten the stop in a direction that is 90 degrees to the plane P of the archwire as shown in FIG. 3. The flattening method attempts to generate a sliding friction between the wire and stop by pressing the tube hard against the wire to create local wire-to-stop contact pressure across the width of the wire as shown in FIG. 4.
For the flattening process to work in a clinically acceptable manner, it is important to maintain a controlled amount of friction between the stop and the wire. Friction is generated by the clamping pressure of the flattened stop across the diameter of the wire as shown in FIG. 4. Though stop flattening is a simple concept, manufacturing experiments have shown that the typical small variations in the dimensions of the wires and stops result in large variations in the friction of the stops on the wires. Large variations in friction result in some stops that either fall off of the archwire or are too tight and cannot be moved easily by the clinician. Other methods of deforming the stops such as notching and pre-crimping (localized indentations as shown in FIG. 5), or partial crimping utilize the same general phenomena (pressure against the width of the wire) to produce local tube-to-wire friction at the site of the notch or crimp and hence are subject to the same unacceptably large variation in friction.
Another shortcoming of the flattening stop approach is that the friction between the stop and wire changes as the stop is moved along the wire. This occurs because the sliding friction is generated by small contact areas between the stop and wire and these small contacts wear off quickly with a small amount of sliding. A stop with adequate initial friction may lose that friction with clinical sliding adjustments. Increases in sliding friction have also been observed, which can be caused by slight increases in wire dimensions (because wire dimensions can change locally within their tolerance range during production processes). Also the stops themselves are often a soft metal and galling (soft metal smearing) can increase friction quickly. Fundamentally, these problems result from the reliance on contact pressure between stop and wire that is directed across the thickness dimensions of the wire. This contact pressure (and resulting friction) changes dramatically with small changes in dimensions in their zones of contact whether due to local wear or wire dimension changes.
There is a need for an improved archwire assembly that eliminates the need for field assembly of stops onto an archwire. An additional need is that a tube placed on the archwire will predictably remain in place during packaging, shipping and installation, but is easily moved to a desired position for final crimping.